Nanofibers are a class of materials that have a variety of uses. These include: fillers for composite materials; electron emitters in vacuum devices; semiconductors, having properties suitable for high performance transistors (better than silicon, gallium-arsenide and other material); structural composites; etc. Typical production methods produce heterogeneous mixtures comprised of superfluous material, conductive and semi-conductive (quasi-insulative) fibers, and a large variability in fiber diameter and length. These materials can be long and highly anisotropic in shape and can be very difficult to handle in a dry state.
Some materials of interest are carbon nanotubes (CNT's). This includes single and multi walled varieties (“species”) as well as conductive (“metallic”) and semiconductive species and also extends to other conducting and semiconducting nanowires and nanofibers.
For example CNT's are grown from the gaseous phase in processes generally called chemical vapor deposition (CVD) and variations thereof. Other manufacturing techniques include the ARC method and laser ablation. Once produced these nanotubes are very small when compared to other particulate matter used in the electronics, structural composite, medical, pharmaceutical manufacturing today. It may be instructive to consider that a “large” single walled CNT may be 10 microns long by 2 nm diameter. By comparison, a “small” dry toner particle for a standard copy machine is usually 8 to 10 microns average diameter, and quite spherical. Recently, high quality color electrostatic printing machines like the NextPress 2100 have started to use “smaller” toner particle of approximately 5 micron diameter which are quite difficult to handle in the dry state and must be suitably coated in order to facilitate handling. When working with very small particles it is often desirable to disperse the particles in a liquid media.
Most of the techniques used to manufacture these nano-materials result in an agglomeration of material. For instance when growing CNT's, one often finds single and/or multi walled species coexisting with other materials such as amorphous carbon, carbon nano-particles and other material. In order to use CNT's for various potential commercial applications it is necessary to separate the various species from one another and to separate those species possessing different characteristics (e.g. electrochemical properties, magnitude of conductivity etc). The process of separation is often referred to as purification. Current purification techniques have only achieved limited success. CNT's have a wide range of properties that make them versatile material for numerous commercial applications. For example CNTs: are excellent conductors; conducting electricity better than copper; conducting heat better than diamonds. They are 100 times stronger than steel at 1/16 the weight and have carrier mobilities 100 times greater than silicon—resulting in better high frequency transistors for electronic parts. They also have desirable magnetic properties. In order to take full advantage of these various properties it becomes important to separate one species of CNT from the other.
Some of the major hurdles confronting the industry are; how to separate conductive (metallic) from semi conductive species, and how to manipulate, align and accurately place these materials in a cost effective manner, particularly given their small size and acicular geometry (0.7 nanometer×1-50 micron). While some manipulation and placement techniques have achieved modest success, there are very few (if any) scalable techniques that have resulted in the ability to align and accurately place one or more nanofibers and/or CNT in an orderly manner at a target location. Current placement techniques include mechanical manipulation via “nano-tweezers”, printing inks via ink jet systems, and dispensing small drops of the nano-material contained in a liquid through pipettes. Most of these techniques have demonstrated at best satisfactory results, are generally expensive, difficult to scale, and/or have performance limitations, for example.
Given the wide range of properties, CNTs are being considered for numerous commercial and industrial applications. CNT related discoveries have spurred other work in semi-conducting nanowires. Nano-wire species of various types can yield superior electronic performance to that of bulk, single crystal material. This has led to the desire to understand and classify these materials as well as discover new ways to manufacture, process, and manipulate them. For example for high performance transistor manufacture and other applications requiring high performance semiconducting parts, nanowires comprised CNT's and other compounds such as Cadmium selenide, Indium arsenide/phosphide, Gallium Indium Phosphide, Gallium Arsenide, Gallium Nitride, and Silicon Carbide among other compounds as well as pure Silicon are applicable to the techniques which we intend to describe.
Another issue that confronts industry is the ability to print useful electrically conductive materials. In general printable conductive inks offer insufficient conductivity while nano powder inks offer insufficient thicknesses to be commercially usefully. For example conductive traces produced with silver filled resin inks produce structures which have about 5% of the conductivity of traces made of equivalent solid metal (e.g. etched aluminum or copper foil). Nano particle inks can achieve 20% to 30% of the conductivity of comparable solid metals but the traces are very thin often less than 500 nanometers in thickness and require a sintering step typically at temperatures of 150 C for approximately 30 minutes. Neither of these solutions would produce sufficient conductivity for most electrical applications. Hybrid circuit technology (e.g. silver filled glasses) can achieve acceptable conductivity but thermal processing often occurring at temperatures ranging from 500 C to 900 C make this technology unsuitable for many different types of substrates.
See, U.S. Pat. No. 6,781,612, and NIP-19 Conference Publication entitled—“Electrostatic Printing of Functional Toner Materials for Electronic Manufacturing Applications”, IS&T NIP Conf, New Orleans, October 2003, expressly incorporated herein by reference.
U.S. Pat. No. 6,781,612 teaches that typical range of toner bath conductivity is of the order 10 to 100 pico mho/cm (10+11 to 10+10 Ω·cm resistivity), and up to 169 pico mho/cm (18 hertz test that measures back and forth flow of electrons, ions, and charged toner particles). A typical toner base is Isopar® G, though Isopar® H and L are also employed. ISOPAR® is the brand name of Exxon for eight grades of high-purity isoparaffinic solvents with narrow boiling ranges.